Uncontrolled growth of microorganisms in industrial production systems can have serious consequences such as lowered product quality, degradation or spoilage of products, contamination of products, and interference with a wide range of important industrial processes. Growth of microorganisms on surfaces exposed to water (e.g., recirculation systems, heat exchangers, once-through heating and cooling systems, pulp and paper process systems, etc.) can be especially problematic, as many of these systems provide an environment suitable for growth of bacteria and other types of microorganisms. Industrial process waters often provide conditions of temperature, nutrients, pH, etc. that allow for growth of microorganisms in the water and on submerged surfaces. Uncontrolled growth of microorganisms is often manifested in the water column with large numbers of free-floating (planktonic) cells as well as on submerged surfaces where conditions favor formation of biofilms.
Biofilm formation is a serious problem in aqueous industrial systems. The first stage of biofilm formation is planktonic cells contacting submerged surfaces either as a result of turbulence in water flow or by active movement toward the surface. If conditions are favorable for growth, microorganisms can attach to the surface, grow, and begin to produce exopolysaccharides that provide three-dimensional integrity to the biofilm. Over time, the biofilm becomes thicker and internally complex as cells reproduce and produce more exopolysaccharides. The microbial community of a biofilm can consist of single or multiple species.
Biofilms are seemingly ubiquitous in all natural, medical, and industrial settings where bacteria exist. Microorganisms can form biofilms on a wide variety of abiotic hydrophobic and hydrophilic surfaces, including glass, metals, and plastics.
Many types of processes, systems, and products can be adversely affected by uncontrolled growth of microorganisms in biofilms and in industrial process waters. Such problems include accelerated corrosion of metals, accelerated decomposition of wood and other biodegradable materials, restricted flow through pipes, plugging or fouling of valves and flow-meters, and reduced heat exchange or cooling efficiency on heat exchange surfaces. Biofilms may also be problematic relative to cleanliness and sanitation in medical equipment, breweries, wineries, dairies and other industrial food and beverage process water systems. Moreover, sulfate-reducing bacteria are often problematic in waters used for the secondary recovery of petroleum or for oil drilling in general. Although sulfate-reducing bacteria can form biofilms on equipment and in pipelines, the significant problem caused by these bacteria is that they generate metabolic by-products that have highly offensive odors, are toxic, and can cause corrosion of metal surfaces by accelerating galvanic action. For example, these microorganisms reduce sulfates present in the injection water to generate hydrogen sulfide, a highly toxic gas that has a highly offensive odor (i.e., rotten egg odor), is corrosive, and reacts with metal surfaces to form insoluble iron sulfide corrosion products.
Paper production is particularly susceptible to adverse effects of biofilms. Paper process waters have conditions (e.g., temperature and nutrients) that favor growth of microorganisms in the water and on exposed surfaces. Biofilms in paper process systems are often referred to as slime or slime deposits and contain paper fiber and other materials used in paper production. Slime deposits can become dislodged from system surfaces and become incorporated into the paper, which results in holes and defects or breaks and tears in the sheet. Such problems result in a lower quality product or unacceptable product being rejected. This necessitates stopping paper production to clean the equipment, which results in the loss of production time.
In order to control problems caused by microorganisms in industrial process waters, numerous antimicrobial agents (i.e., biocides) have been employed to eliminate, to inhibit or to reduce microbial growth. Biocides are used alone or in combination to prevent or control the problems caused by growth of microorganisms. Biocides are usually added directly to a process water stream typical method of addition is such that the biocide is distributed throughout the process system. In this manner, planktonic microorganisms and those in biofilms on surfaces in contact with the process water can be controlled.
Many organic and inorganic substances are used as biocides in industrial process systems. The type of biocide used in a given system will depend on many factors including, but not limited to, the nature of the medium to which the biocide is added, the problematic microorganism(s), as well as specific requirements of the industry, including safety and regulatory considerations.
Depending on their chemical composition and mode-of-action, biocides are classified as oxidizing or non-oxidizing. Oxidizing and non-oxidizing biocide can be used alone or in combination depending on the application. Oxidizing biocides have been widely used in industry for decades, especially in pulp and paper production where strong oxidizers have been used to control microbial populations. Oxidizing biocides such as chlorine gas, sodium hypochlorite, hypobromous acid, and chlorine dioxide are widely used as biocides to treat recirculating waters in many types of industries. Two of the primary reasons for using these and other oxidizing biocides is that such oxidizers are: (1) inexpensive; and (2) non-specific regarding which types of microorganisms are inhibited; if sufficient concentrations of oxidizing biocides are achieved virtually all microorganisms can be inhibited.
Of the oxidizing biocides, chlorine is the most widely used to treat recirculating water systems. The chemistry of chlorine well known. When added to water, chlorine can exist in either of two forms, HOCl and OCl−, depending on pH. These chemical species of chlorine, also referred to as “free chlorine,” react with a wide variety of compounds in aqueous systems.
The highly reactive nature of chlorine may also be a liability, as some of the oxidizer will be used (e.g., consumed) during reactions with non-biological material. Therefore, in order to provide enough oxidizer to react with microorganisms in a process stream, the total amount of oxidizer needed to inhibit microorganisms will include that used in reactions with non-biological components of the system. Reactions with non-biological components of process water not only add to treatment cost, but undesired by-products can be generated and other additives in the process stream can be adversely affected.
Process streams such as in paper mills are especially problematic for highly reactive oxidizers because of the high concentrations of dissolved and particulate inorganic and organic materials. Such process waters exhibit a very high “demand” on the oxidizer. “Demand” is defined as the amount of chlorine that reacts with substances other than the target microorganisms in the process water. In order to maintain an effective concentration of chlorine in an aqueous system to inhibit microorganisms, an amount in excess of the demand must be applied. The types and amounts of inorganic and organic materials in a process stream will define the demand for an oxidizer. For example, many substances are known to react with chlorine and result in the chlorine being non-biocidal; such substances include sulfides, cyanides, metal ions, lignin, and, among others, various water treatment chemicals (e.g., some scale and corrosion inhibitors).
Although effective as biocides, strong oxidizers such as sodium hypochlorite can cause many problems in an industrial process stream such as increased corrosion rates, increased consumption of wet end additives, and, among others, decreased life of felts used on paper machines.
Because of the inherent reactivity of chlorine and related strong oxidizers with non-biological organic and inorganic materials, it is desirable to have the oxidizer in a form that would have antimicrobial activity but be less reactive with non-biological materials. Therefore, the process of chloramination has been used to avoid some of the problems associated with the use of strong oxidizers. Chloramination can occur by either (1) adding chlorine to a water system that contains a known, low concentration of ammonia, or (2) adding ammonia to a water system that contains a known, low concentration of chlorine. In either situation, the chlorine and ammonia will react in situ to form a chloramine. Chloramines generated from reacting chlorine and ammonia include monochloramine (NH2Cl), dichloramine (NHCl2), and trichloramine (NCl3). Two of the important parameters that determine which chloramine species will exist in a system are pH and the ratio of Cl to N.
Chlorine, as a gas or liquid, and ammonia are commonly combined to form chloramines. However, other substances containing an amine (RNH2) group can also form chloramines. The antimicrobial activity of a chloramine depends on the chemical nature of the amine-containing compound. For example, ammonium hydroxide can react with an oxidizing halogen donor such as sodium hypochlorite to form monochloramine; this chloramine will be an effective biocide. However, if an amino acid, such as glycine (NH2CH2COOH) is reacted with sodium hypochlorite, the amine group will be chlorinated, forming a mono- or di-chloramine species. The chlorinated glycine has less antimicrobial activity compared to monochloramine generated from ammonium hydroxide.
Chloramines are attractive for water treatment because of their stability in situ, ease of application and monitoring, and low capital and operational costs. Monochloramine is the preferred chemical species for disinfecting a water supply. Dichloramine is reported to be a superior disinfectant but has negative properties such has high volatility and odor.
The difference in reactivity and specificity of chlorine and monochloramine may allow the latter to penetrate a biofilm and react with the denizens whereas the former is consumed in non-specific reactions with materials in the water or abiotic components of the biofilm before it fully penetrates the biofilm.
Monochloramine is used as a single active to treat water for controlling growth of microorganisms in water and wastewater systems. Studies have shown that the pH of an aqueous system affects efficacy of monochloramine; the efficacy increases as pH decreases. Other physical and chemical parameters of a system can affect efficacy of chloramines by influencing the stability of the compounds. Parameters such as pH, temperature, and the presence of other chemicals have influence on the stability of monochloramine in water; at pH 7.5, the half-life of monochloramine is about 75 hours at 35 C but it is greater than 300 hours at 4 C.
Although widely practiced for treating municipal water distribution systems, chloramines are not commonly used in industrial systems. Chlorine (in bleach or chlorine gas) was used in combination with ammonia in papermaking systems. There was a shift toward using other oxidizing and non-oxidizing biocides in papermaking systems in subsequent years. However, recently there appears to be renewed interest in using chloramines in papermaking systems (see U.S. Pat. Nos. 6,478,973; 6,132,628; 5,976,386, the contents of each is herein incorporated by reference). For example, it has been shown that ammonium bromide reacted with sodium hypochlorite produces an effective biocide for industrial applications (U.S. Pat. No. 5,976,386, the content of which is herein incorporated by reference). Furthermore, this biocide is especially effective for controlling problems associated with microbial growth in pulp and paper process waters that have a pH in the alkaline range. The biocide generated from ammonium bromide, reported by Barak as “bromide-activated chloramine,” effectively reduces the total microbial community within a system (i.e., biofilm-associated as well as planktonic bacteria) where the pH is neutral to alkaline. Barak teaches that the preferred pH of the receiving water should be in the range of 7 to 9; the biocide is effective in alkaline paper process water but does not interfere with other pulp and paper process and functional additives (e.g., wet and dry strength additives, size agents, dyes, etc), unlike other common oxidizer programs.
There remains a need for improved biocides that are effective under harsh environmental conditions such as found in the papermaking industry and other industrial processes.